The traditional telecommunications network operated by the telephone companies uses circuit switching and time division multiplexed (TDM) transmission. A TDM channel is characterized by the use of fixed time periods known as frames. Each frame in turn is divided into a fixed number of timeslots. With TDM, a fixed amount of bandwidth is dedicated through the network for the duration of a circuit switched connection. This bandwidth is fixed such that even if there is no data or voice being transmitted for a period of time, resources of the network are used to support the connection.
High speed transmission over fiber-optic networks uses a synchronous hierarchy of TDM transmission rates known as Synchronous Optical Network (SONET). This Bellcore-defined standard established a set of data rate and framing standards for data transmission using optical signals over fiber-optic cables. The SONET data rate and framing standards are designated as Synchronous Transport Signal (STS-n) levels; the corresponding SONET optical signal standards are designated as Optical Carrier (OC-n) levels. For the STS level, xe2x80x9cnxe2x80x9d represents the level at which the respective data rate is exactly xe2x80x9cnxe2x80x9d times the first level. For example, STS-1 has a defined data rate of 51.84 Mbps; thus, STS-3 is three times the data rate of STS-1, or 3xc3x9751.84=155.52 Mbps. Corresponding to each data rate is an equivalent optical fiber rate. For example, the OC-1 fiber rate corresponds to STS-1, and OC-3 corresponds to STS-3.
In a typical SONET application, as shown in the block diagram of FIG. 1, add/drop multiplexers (ADMs) 12 are connected in a logical ring 14. For a point-to-point connection between source and destination endpoints 10a and 10b, respectively, each endpoint 10a, 10b is allocated a fixed number of timeslots at both the ingress and egress ADMs 12a, 12b. For example, the endpoints 10a, 10b are shown with an allocation of an OC-3c rate which is defined as 155.52 Mbps in the SONET hierarchy.
To setup or provision such a connection, the network operator configures the SONET transmission facilities using information about the source and destination endpoints, e.g., node, slot, port (and optionally timeslots) at the ingress and egress ADMs 12a, 12b. A primary advantage of this TDM scheme is that connections are very easy to configure and manage. All the operator needs to know is the port to port connectivity required. The operator does not need to know about the particular traffic being carried within the connection in order to setup the connection through the network.
There are, however, several drawbacks with the TDM approach. One drawback is that it does not provide for any statistical multiplexing gains when there is no actual traffic on a particular established connection. This means that each connection needs to have dedicated bandwidth which, once configured, does not allow for sharing of network resources.
Another drawback is that the ingress and egress ports have to match each other in allocated bandwidth. For example, an OC-3c port on ingress can only be connected to an OC-3c port or an OC-3 within an OC-12 on egress. That is, a connection cannot be made between an OC-3c on ingress and, for example, an OC-12c on egress.
A further drawback of the TDM approach is that bandwidth can only be provisioned in increments defined by the TDM hierarchy, e.g., the aforementioned SONET rates or the traditional hierarchy of DS-n rates. The TDM approach does not provide for flexible data rates between these discrete TDM increments.
A cell relay approach, such as defined for asynchronous transfer mode (ATM) networks, avoids the aforementioned drawbacks. ATM networks use fixed-length cells to provide connection-oriented data transmission in which multiple traffic types (e.g., voice, data and video) are asynchronously combined on a communication channel such that the transmission bandwidth on the channel can be allocated as needed to the multiple traffic types. That is, bandwidth is flexible. ATM is a simple, fast, cell-switching technology which derives its cell routing capabilities from information carried within a header in each cell.
In ATM, there are two types of interfaces defined: the user-network interface (UNI) for use between an endpoint and an ATM switch and the network-node interface (NNI) for use between two ATM switches. The standard ATM cell format for the UNI is shown in FIG. 2. An ATM cell contains 53 octets and has two parts: a 5-octet header and a 48-octet payload. The header includes the following fields: generic flow control (GFC); virtual path identifier (VPI); virtual channel identifier (VCI); payload type (PT); cell loss priority (CLP); and header error check (HEC). The VPI and VCI fields together represent an ATM address that is used for mapping incoming VPI/VCI to outgoing VPI/VCI based on the connection type (permanent virtual circuit or switched virtual circuit). The standard ATM cell format for the NNI has no GFC field in the header and a larger VPI field.
There are two types of connections in an ATM network: virtual paths, identified by a VPI in the cell header, and virtual channels, identified by the combination of a VPI and a VCI. A virtual path is a collection of virtual channels which are switched together across the ATM network according to the same VPI. A transmission link is a collection of virtual paths and virtual channels. FIG. 3 shows this relationship among virtual channels, virtual paths and the transmission link.
ATM cells can be carried within the synchronous payload envelope of a SONET frame as shown in FIG. 4. The STS-3c frame includes a SONET framing signal 20 and a payload section that accommodates 44 ATM cells generally labeled 22. Because the cell rate is not an integer number of the frame rate, the last cell 22-x1 is continued in the next frame as cell 22-x2. The cell boundaries change from frame to frame and only repeat with each fifty-third frame. The flexible bandwidth nature of ATM is evident by noting that succeeding cells 22-a, 22-b have differing VPI/VCI information.
The basic ATM switching operation is described briefly. A cell is received at an incoming port of an ATM device. The received cell includes incoming VPI and VCI values in its header. The ATM device looks up the incoming VPI/VCI values in a local translation table which correlates the incoming port and VPI/VCI values with an outgoing port and outgoing VPI/VCI values. The incoming VPI/VCI values are replaced with the outgoing VPI/VCI values in the cell header and the cell is then transmitted to the next ATM device in the network connection through the corresponding outgoing port. This cell processing is repeated at each succeeding ATM device along the connection until the cell reaches its destination. Since the VPI/VCI have only local significance across a particular link in the end-to-end connection, it is clear then that each cell enters and exits the end-to-end connection with different VPI/VCI values.
One drawback with traditional methods of provisioning point-to-point ATM connections is that at each ATM device along the connection, the network operator has to configure the incoming and outgoing VPI/VCI information in the translation table. Alternatively, the ATM cells can be transported together transparently in a TDM xe2x80x9cpipexe2x80x9d such as a SONET OC-3c circuit. However, this TDM approach to transport of ATM is bandwidth inefficient when the circuit is not fully loaded.
A need exists for an approach to ATM provisioning of point-to-point connections that emulates the configuration ease and transparency of a TDM pipe but is bandwidth efficient. A need also exists for a provisioning approach that does not require the operator to configure each incoming and outgoing VPI/VCI for the ATM devices in a network connection.
In accordance with the present invention, a novel mechanism for providing point-to-point connections converts VPI/VCI information at an ingress ATM device on incoming cells from a source endpoint to internal VP/VCI information that is used to switch the cells through an ATM network to an egress ATM device. At the egress ATM device, the internal VPI/VCI information is converted back to the source VPI/VCI information for transmission of cells to the destination endpoint. In such a manner, an ATM switch network is able to provide point-to-point connections without requiring individual configuration of VPI/VCI information at the ingress and egress ATM devices.
According to the present invention, a method for providing a connection between a source endpoint and destination endpoint through an ATM network comprises providing a network of plural ATM switching nodes, including an ingress node and an egress node, the ingress node having a receiving port for receiving ATM cells from a source endpoint, and the egress node having a transmitting port for transmitting ATM cells to a destination endpoint. At the ingress node, source ATM cells are received, with each ATM cell having a header that includes source virtual path identifier/virtual circuit identifier (VPI/VCI) information. The source VPI/VCI information of each received source ATM cell is mapped to internal VPI/VCI information to provide internal ATM cells that are switched through the network on a virtual path to the egress node. The virtual path corresponds to the internal VPI/VCI information. At the egress node, the internal VPI/VCI information of each internal ATM cell is mapped to the source VPI/VCI information for transmission to the destination endpoint.
According to an aspect of the invention, mapping of the source VPI/VCI information at the ingress node includes mapping a source VPI and a source VCI to an internal VCI by selecting N bits of the source VCI; storing the N selected bits in a register; selecting M bits of the source VPI; storing the M selected bits in the register to the left of the N bits to provide M+N bits; and replacing the VCI field of the source ATM cell with the M+N bits from the register.
According to another aspect of the invention, mapping of the internal VPI/VCI information at the egress node includes mapping the internal VCI to the source VPI and the source VCI by selecting M bits from bit positions N+1 to N+M of the internal VCI; replacing the VPI field of the internal ATM cell with the M selected bits; selecting N bits from bit positions 1 to N of the internal VCI; and replacing the VCI field of the internal ATM cell with the N selected bits.
According to a further aspect of the invention, a mechanism for providing point-to-multipoint and multipoint-to-multipoint connections maps the source VPI/VCI information of each received source ATM cell at an ingress node to internal VPI/VCI information having information indicating a particular egress node. Internal ATM cells are switched through the network to the particular egress node indicated by the internal VPI/VCI information. At each particular egress node, the internal VPI/VCI information of each internal ATM cell is mapped to the source VPI/VCI information for transmission to the corresponding destination endpoint. The mapping of the source VPI/VCI information includes mapping a source VPI and a source VCI to an internal VCI and mapping a portion of the source VPI to an internal VPI indicating a particular egress node.
According to an alternate embodiment of the invention, mapping of the source VPI/VCI information at the ingress node includes mapping a source VPI and a source VCI to an internal VPI and internal VCI, respectively. A mapping information message is transmitted from the ingress node to the egress node indicating the mapping at the ingress node. The mapping information message is received at the egress node and the internal VPI and VCI are mapped to the source VPI and VCI, respectively, according to the mapping indicated by the mapping information message.
The term xe2x80x9cpoint-to-pointxe2x80x9d is used herein to refer to a connection from one source endpoint to a single destination endpoint. Within that point-to-point connection there can be ATM cells having different VPI and VCI values.
The term xe2x80x9cpoint-to-multipointxe2x80x9d is used herein to refer to a connection from one source endpoint to multiple destination endpoints. In a point-to-multipoint connection as used herein, ATM cells from a source endpoint are routed to a destination endpoint based on the VPI value contained within the cell.
The term xe2x80x9cmultipoint-to-multipointxe2x80x9d is used herein to refer to a mesh network having multiple source endpoints connected to multiple destination endpoints.
It should be noted that, while the preferred embodiments are described in the context of ATM cell relay, the principles of the invention are also applicable to multi-protocol label switching.